1. Field of the Invention
The invention relates to electronic targets for acquiring light beams used as references in survey and construction.
2. Description of the Prior Art
Targets for acquiring reference beams are commonly adapted for operation in two phases. Phase one is rough beam acquisition. Phase two is precision centering. In my prior application, Ser. No. 427,372 filed Dec. 21, 1973, now U.S. Pat. No. 3,857,639, an electronic target was disclosed in which a lens of relatively large aperture was utilized for rough acquisition and then the aperture was masked with a slot for precision centering. U.S. Pat. No. 3,588,249 to Studebaker and U.S. Pat. No. 3,469,919 to Zellner both describe targets with effectively two detectors which are rough tuned when one detector is activated and are precisely centered by adjusting for a null or the like on an indicating instrument indicating equal signals from both detectors. The former arrangement entails extra cost in optics while the latter requires either matched or compensated detectors.